`--sampling-step` only support integer values

[japdlsd]

Hello :)

An attempt to run GraphChainer with non-integer --sampling-step parameter raises an error:

the argument ('0.1') for option '--sampling-step' is invalid

which clashes with its description in README.md:

--sampling-step Sampling step factor (default 1). Use >1 (<1, >0) for faster (slower), but less (more) accurate alignments. It increases (decreases) the sampling sparsity of fragments.